Re: [RFC PATCH 3/3] x86/cpufeature: Remove unused and seldomly used cpu_has_xx macros

From: Josh Triplett
Date: Tue Nov 24 2015 - 17:42:37 EST

On Tue, Nov 24, 2015 at 02:05:10PM +0100, Borislav Petkov wrote:
> On Tue, Nov 10, 2015 at 12:48:42PM +0100, Borislav Petkov wrote:
> > From: Borislav Petkov <bp@xxxxxxx>
> >
> > Those are stupid and code should use static_cpu_has_safe() anyway. Kill
> > the least used and unused ones.
> >
> > Signed-off-by: Borislav Petkov <bp@xxxxxxx>
> > Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> > Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > Cc: Matt Mackall <mpm@xxxxxxxxxxx>
> > Cc: Chris Mason <clm@xxxxxx>
> > Cc: Josef Bacik <jbacik@xxxxxx>
> > Cc: David Sterba <dsterba@xxxxxxxx>
> > ---
> > arch/x86/crypto/chacha20_glue.c | 2 +-
> > arch/x86/crypto/crc32c-intel_glue.c | 3 ++-
> > arch/x86/include/asm/cmpxchg_32.h | 2 +-
> > arch/x86/include/asm/cpufeature.h | 32 +++--------------------------
> > arch/x86/include/asm/smp.h | 2 +-
> > arch/x86/kernel/cpu/amd.c | 2 +-
> > arch/x86/kernel/cpu/intel.c | 3 ++-
> > arch/x86/kernel/cpu/mtrr/generic.c | 2 +-
> > arch/x86/kernel/cpu/mtrr/main.c | 2 +-
> > arch/x86/kernel/cpu/perf_event_amd.c | 4 ++--
> > arch/x86/kernel/cpu/perf_event_amd_uncore.c | 8 ++++----
> > arch/x86/kernel/fpu/init.c | 4 ++--
> > arch/x86/kernel/hw_breakpoint.c | 3 ++-
> > arch/x86/kernel/vm86_32.c | 4 +++-
> > arch/x86/mm/setup_nx.c | 4 ++--
> > drivers/char/hw_random/via-rng.c | 5 +++--
> > drivers/crypto/padlock-aes.c | 2 +-
> > drivers/crypto/padlock-sha.c | 3 ++-
> > fs/btrfs/disk-io.c | 2 +-
> > 19 files changed, 35 insertions(+), 54 deletions(-)
>
> Ok, 0day says this patch makes tiny not so tiny:
>
> i386-tinyconfig vmlinux size:
>
> +-------+------+-------+-----+--------------------------------------------------------------------------------------+
> | TOTAL | TEXT | DATA | BSS | |
> +-------+------+-------+-----+--------------------------------------------------------------------------------------+
> | +4646 | +64 | +4096 | 0 | ab9976b5af96 x86/cpufeature: Remove unused and seldomly used cpu_has_xx macros |
> | -32 | -32 | 0 | 0 | 13e835020a02 x86/cpufeature: Cleanup get_cpu_cap() |
> | +32 | +32 | 0 | 0 | 3615f94f0486 x86/cpufeature: Move some of the scattered feature bits to x86_capabili |
> | +136 | +32 | 0 | 0 | 506d983184f4 Merge branch 'tip-fpu-xsave' into rc2+ |
> | +4782 | +96 | +4096 | 0 | ALL COMMITS |
> +-------+------+-------+-----+--------------------------------------------------------------------------------------+
>
> Btw, thanks 0day!

Yay, it worked! Thanks for paying attention to this.

> The problem comes from static_cpu_has_safe() adding the alternatives and
> fallback machinery. For example, before the patch, we had this at the
> cpu_has_* testing sites:
>
> movl boot_cpu_data+20, %eax # MEM[(const long unsigned int *)&boot_cpu_data + 20B], D.19113
> testl $2097152, %eax #, D.19113
> je .L166 #,
>
> and now we get this:
>
> #APP
> # 449 "arch/x86/kernel/cpu/intel.c" 1
> # 0 "" 2
> # 511 "./arch/x86/include/asm/cpufeature.h" 1
> 1: jmp .L166 #
> 2:
> .skip -(((5f-4f) - (2b-1b)) > 0) * ((5f-4f) - (2b-1b)),0x90
> 3:
> .section .altinstructions,"a"
> .long 1b - .
> .long 4f - .
> .word 117 #
> .byte 3b - 1b
> .byte 5f - 4f
> .byte 3b - 2b
> .previous
> .section .altinstr_replacement,"ax"
> 4: jmp .L167 #
> 5:
> .previous
> .section .altinstructions,"a"
> .long 1b - .
> .long 0
> .word 21 #
> .byte 3b - 1b
> .byte 0
> .byte 0
> .previous
>
> # 0 "" 2
> #NO_APP
> jmp .L168 #
> .L166:
> movl $21, %eax #,
> call __static_cpu_has_safe #
> testb %al, %al # D.19126
> je .L167 #,
> .L168:
> #APP
> # 453 "arch/x86/kernel/cpu/intel.c" 1
> # 0 "" 2
> #NO_APP
>
> That gets spread among .altinstructions, .altinstr_replacement, .text
> etc sections. .data grows too probably because of the NOP padding :-\

Yeah, padding makes the evaluation of some section sizes painful.

That said: .data? I don't quite see how that happened.

> text data bss dec hex filename
> before: 644896 127436 1189384 1961716 1deef4 vmlinux
> after: 645446 131532 1189384 1966362 1e011a vmlinux
>
> [Nr] Name Type Addr Off Size ES Flg Lk Inf Al
> before: [12] .altinstructions PROGBITS c10bdf48 0bef48 000680 00 A 0 0 1
> after: [12] .altinstructions PROGBITS c10bff48 0c0f48 0007d2 00 A 0 0 1
>
> before: [13] .altinstr_replace PROGBITS c10be5c8 0bf5c8 00016c 00 AX 0 0 1
> after: [13] .altinstr_replace PROGBITS c10c071a 0c171a 0001ad 00 AX 0 0 1
>
> before: [ 7] .data PROGBITS c1092000 093000 0132a0 00 WA 0 0 4096
> after: [ 7] .data PROGBITS c1093000 094000 0142a0 00 WA 0 0 4096
>
> So I'm wondering if we should make a config option which converts
> static_cpu_has* macros to boot_cpu_has()? That should slim down
> the kernel even more but it won't benefit from the speedup of the
> static_cpu_has* stuff.
>
> Josh, thoughts?

Seems like a good idea to me: that would sacrifice a small amount of
runtime performance in favor of code size. (Note that the config option
should use static_cpu_has when =y, and the slower, smaller method when
=n, so that "allnoconfig" can DTRT.)

Given that many embedded systems will know exactly what CPU they want to
run on, I'd also love to see a way to set the capabilities of the CPU at
compile time, so that all those checks (and the code within them) can
constant-fold away.

- Josh Triplett
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